This invention relates to systems for comparing, measuring or providing a reference signal based on variable oscillator loop gain and is described. Only when the oscillator loop gain is at least one does the oscillator produces an AC signal. The oscillator ability to oscillate is controlled by the one or more sensor or transducer input signal levels. Also in some cases negative feedback of the oscillator's AC signal is used to control the loop gain of the oscillator circuit, keeping the loop gain close to or at the value of one. The system's output signal depends on whether the oscillator is oscillating or not, or the oscillator's AC signal level required to just maintain oscillation.
Transducers and sensors have often been used to provide the input signals for many forms of prior art transducer instrumentation, for example, scales or balances, accelerometers, and pressure transducers or proximity gauges. In such systems, the precision at which measurements can be made is very much a function of the stability of the circuit interfacing the sensor portion and read-out-portion of the system. In general, the prior art systems have permitted relatively low precision measurements due to the sensitivity of the interface circuit to combinations of various factors such as drive signal waveshape, drive signal amplitude, drive signal frequency, and temperature dependent component parameter variation.
More particularly, the class of system exemplified by U.S. Pat. No. 3,318,153 to Lode includes circuit interface which generates an output signal as derived from a rectification and summing of current signals whose amplitude is dependent on both the magnitude and frequency of an applied drive signal, thereby requiring a high voltage drive signal at relatively low frequencies and establishing a relatively large power requirement. Thus, that system has a relatively high sensitivity to both drive signal amplitude and frequency, and, as a result, means is provided by Lode to maintain the amplitude-frequency product for the drive signal to be constant.
An other application of this invention is as a signal reference system. Stable references are required for circuits such as A/D converters, measurement devices, and voltage regulators to name only a few.
A buried-Zener reference is one way to produce a reference voltage. Another way to produce a reference voltage is with a bandgap voltage circuit. Bandgap voltage circuits can operate with a lower supply voltage than buried-Zener references and can also require less power. Operation of a bandgap voltage reference is well-known and produces a reference voltage that corresponds to the bandgap voltage of the used substrate material. In the most common case of silicon, this bandgap voltage is approximately 1.2 volt. The conventional bandgap reference circuits can therefore not be used for generating the necessary reference voltage in systems with supply voltages of 1.5 volt and below.
An objective of the present invention is to provide a system for comparing, measuring or providing a reference signal that has a high temperature, radiation, and voltage stability due to its reliance on passive component ratios to set circuit thresholds operating values. Passive components such as resistors and capacitors are more stable under these conditions. This invention increases system accuracy by making the accuracy dependent on passive component ratios instead of transistor or signal frequency stability.
An other objective of the present invention to provide a system for providing an output signal level dependant on the impedance or gain associated with a transducer or sensor using a system being relatively independent of the system signal amplitude, frequency, and waveshape. Additionally the system is relatively simple and inexpensive to implement.
An other objective of the invention is to provide a reference voltage or current which is insensitive to temperature and to variations in the main power supply and yet can produce a voltage at less than the bandgap voltage.
A further objective of the invention to provide circuits that are less susceptible to process variances by relying on impedance ratios thereby providing a more consistently manufacturable circuit.